The earliest polyolefin prepared industrially is polyethylene. Several types of polyethylene exist and are generally classified according to their density.
The high density polymer (HDPE, for High Density PolyEthylene), with a density generally of between 0.940 and 0.965 g/cm3; this polyethylene is distinguished by a low degree of branching and consequently by strong intermolecular forces and by a high tensile strength. The low branching is provided by the choice of the catalyst and of the reaction conditions.
The medium density polymer (MDPE, for Medium Density PolyEthylene), with a density generally of between 0.925 and 0.940 g/cm3; this polyethylene exhibits good impact properties.
The low density polymer (LDPE, for Low Density PolyEthylene), with a density generally of between 0.915 and 0.935 g/cm3; this polymer exhibits a high degree of branching of chains (short and long). This polyethylene exhibits a low tensile strength and an increased ductility.
The linear low density polymer (LLDPE, for Linear Low Density PolyEthylene), with a density generally of between 0.915 and 0.935 g/cm3; this polymer exists in a substantially linear form with a large number of short branches.
The very low density polymer (VLDPE, for Very Low Density PolyEthylene), with a density generally of between 0.860 and 0.910 g/cm3; this polymer exists in a substantially linear form with a very large number of short branches.
In addition, there exists subcategories when the polyethylene is crosslinked or also according to its molecular weight.
Furthermore, the polyethylene is often used in combination with a second material other than polyethylene. It is possible to produce, for example, multilayer films comprising at least one layer of polyethylene and at least one other layer of this second material. Mention may be made, as example of second material, of polar polymers and also metals, alloys of metals or their oxides. Mention may be made, as polar polymer, of nitrogen-comprising and/or oxygen-comprising polymers, such as a polyamide, a saponified copolymer of ethylene and of vinylacetate or a polyester. However, these materials do not adhere or adhere only slightly to polyethylene. It is therefore necessary to use a layer of intermediate “tie” which adheres to the layers of polyethylene and to the second layer in order to be able to combine these 2 layers in the multilayer film. While polyethylenes of renewable origin have been described in the prior art, such as, for example, in the document US 2007/0219521, there currently exists no tie produced from renewable starting materials which makes it possible to combine a layer of polyethylene with a layer of a second material.
Advantageously and surprisingly, the inventors of the present patent application have employed a process for the industrial manufacture, from renewable starting materials, of a specific polyethylene which is a grafted polyethylene of use in tie manufacture.
The process according to the invention makes it possible to dispense, at least in part, with starting materials of fossil origin and to replace them with renewable starting materials.
In addition, the polyethylene obtained according to the process according to the invention is of such a quality that it can be used in any application in which it is known to use grafted polyethylene, including in the most demanding applications.
In particular, it makes it possible to manufacture a tie from renewable starting materials which makes it possible to combine a layer of polyethylene with a second material chosen from polar polymers and also metals, alloys of metals or their oxides.